Experimental characterization of a multi-effect distillation system coupled to a flat plate solar collector field: Empirical correlations

In the last three decades, developments in desalination plants have been focused on the minimization of its energetic consumption and cost. Advancements include emerging technologies that make use of low grade thermal energy, like Multi-Effect Distillation (MED). The energy optimization of MED systems and their coupling with solar thermal technologies have been deeply investigated by the Solar Desalination Unit of the Plataforma Solar de Almería (PSA) through a solar desalination test facility consisting of a MED plant coupled to a static solar field. Recently, the previous solar field composed of compound parabolic concentrators (CPC) has been replaced by a new one with large-aperture flat plate collectors (FPC). In this work, an experimental characterization of the solar MED system under off-design conditions is presented and discussed. The efficiency of the FPCs' solar field at several temperature levels for different climate conditions has been determined. Also, the influence of the variation of key parameters by which the MED plant is controlled (the inlet hot water flow rate (mhot) and temperature (Thot), the feed water flow rate (mf) and the condenser vapor temperature (Tc)) on the freshwater production and performance ratio (PR), were analyzed with an experimental campaign of 82 experiments. The results obtained were used to fit polynomial expressions, validated statistically, that predict the distillate yield and the PR for different operation strategies. The experimental characterization of the desalination plant revealed that the rise in the Tc has the highest impact in the thermal efficiency of the MED unit, and the rise in the mf has the greatest influence on the distillation production. Moreover, from the analysis of the coupling of the MED plant with the solar field it was found that the thermal storage has an autonomy of up to 6 h when the MED works with hot water at 65 °C, which allows the plant to continue the operation even in transient periods (presence of clouds). Also, from a theoretical annual study for the solar MED system, it was found that the annual solar fraction ranges from 30.7% to 42.4%, which leads to an annual distillate production between 6701 m3 and 11,314 m3, respectively.